The present invention generally relates to noise reduction circuits for a video signal, and more particularly to a noise reduction circuit which carries out a signal processing so as to effectively reduce a noise without deteriorating the waveform of an original video signal.
Generally, a video information signal of a television video signal has a correlation substantially in a vertical direction of a picture, and there is a correlation between mutually adjacent horizontal scanning lines. Such a correlation is generally referred to as a vertical correlation or a line correlation. However, a noise component exists at random, and does not have a vertical correlation as in the case of the video information signal.
Thus, there was a conventional noise reduction circuit which was designed to use the fact that there is the vertical correlation in the video information signal and no vertical correlation in the noise component, in order to reduce the noise. This conventional noise reduction circuit was made up of a delay circuit for delaying an input video signal by one horizontal scanning period (1H), and an adding circuit for adding an output delayed video signal of the delay circuit and the input video signal which has not been delayed. When the delayed video signal and the undelayed video signal are added in the adding circuit, the level of the signal component is doubled by the addition because there is the vertical correlation in the video information signal. However, since there is no vertical correlation in the noise component, the energy of the noise component is root-mean-squared by the addition. In other words, the level of the noise component becomes .sqroot.2 times the original level before the addition. Accordingly, when the level of the signal component after the addition is returned to the original level before the addition, the noise component becomes 1/.sqroot.2 times the original level, and the signal-to-noise (S/N) ratio is improved by 3 dB.
However, although there generally is the vertical correlation in the video signal, there is no vertical correlation before and after a point where the waveform of the video signal undergoes a rapid change (for example, in an extreme case, a point where the video signal changes from white to black). In this case, the level of the video signal does not become doubled in the output video signal of the above adding circuit which adds the delayed video signal and the undelayed video signal, at parts of the video signal where the vertical correlation does not exist. Thus, stepped parts are introduced in the waveform at the parts where the vertical correlation does not exist, and the output video signal waveform of the adding circuit becomes different from the original video signal waveform. The part of the video signal waveform where there is no vertical correlation, which becomes different from the original video signal waveform as a result of the signal processing which is carried out to reduce the noise, will be referred to as a correlation error in the present specification. When the correlation error occurs, the luminance and color at the part of the video signal where the correlation error has occurred, become different from the luminance and color at a corresponding part of the original video signal. Therefore, the conventional noise reduction circuit was disadvantageous in that it was impossible to obtain a satisfactory reproduced picture although the noise could be reduced.